[#1689] client/netmap: Refactor Client.config()

There are problems with that code:
- explicit casts,
- `ErrConfigNotFound` which is not a part of a public API,
- hand-rolled assertions, even though neo-go already has everything we
  need.

So, remove the error, use `stackitem/Item.Try*()` methods for
conversions. Note, that readUint64Config() returns an error if the
parameter is missing. This is likely an error, but this behaviour is
preserved in this PR: `TryInteger()` returns error when applied to
`Null`. By contract, `TryBool()` returns false for `Null`, so this
PR introduces no functional changes.

Refs 82c7a50b8a/pkg/vm/stackitem/item.go (L418)

Change-Id: I445d28a7c6b5abb9a2bb97b57c0cc42d617e16f7
Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

pkg/morph/client/netmap/config.go

@@ -2,7 +2,6 @@ package netmap
 
 import (
 	"context"
-	"errors"
 	"fmt"
 
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/morph/client"
@@ -106,29 +105,27 @@ func (c *Client) MaintenanceModeAllowed(ctx context.Context) (bool, error) {
 }
 
 func (c *Client) readUInt64Config(ctx context.Context, key string) (uint64, error) {
-	v, err := c.config(ctx, []byte(key), IntegerAssert)
+	v, err := c.config(ctx, []byte(key))
 	if err != nil {
 		return 0, fmt.Errorf("read netconfig value '%s': %w", key, err)
 	}
 
-	// IntegerAssert is guaranteed to return int64 if the error is nil.
-	return uint64(v.(int64)), nil
+	bi, err := v.TryInteger()
+	if err != nil {
+		return 0, err
+	}
+	return bi.Uint64(), nil
 }
 
 // reads boolean value by the given key from the FrostFS network configuration
 // stored in the Sidechain. Returns false if key is not presented.
 func (c *Client) readBoolConfig(ctx context.Context, key string) (bool, error) {
-	v, err := c.config(ctx, []byte(key), BoolAssert)
+	v, err := c.config(ctx, []byte(key))
 	if err != nil {
-		if errors.Is(err, ErrConfigNotFound) {
-			return false, nil
-		}
-
 		return false, fmt.Errorf("read netconfig value '%s': %w", key, err)
 	}
 
-	// BoolAssert is guaranteed to return bool if the error is nil.
-	return v.(bool), nil
+	return v.TryBool()
 }
 
 // SetConfigPrm groups parameters of SetConfig operation.
@@ -277,15 +274,11 @@ func bytesToBool(val []byte) bool {
 	return false
 }
 
-// ErrConfigNotFound is returned when the requested key was not found
-// in the network config (returned value is `Null`).
-var ErrConfigNotFound = errors.New("config value not found")
-
 // config performs the test invoke of get config value
 // method of FrostFS Netmap contract.
 //
 // Returns ErrConfigNotFound if config key is not found in the contract.
-func (c *Client) config(ctx context.Context, key []byte, assert func(stackitem.Item) (any, error)) (any, error) {
+func (c *Client) config(ctx context.Context, key []byte) (stackitem.Item, error) {
 	prm := client.TestInvokePrm{}
 	prm.SetMethod(configMethod)
 	prm.SetArgs(key)
@@ -301,26 +294,7 @@ func (c *Client) config(ctx context.Context, key []byte, assert func(stackitem.I
 			configMethod, ln)
 	}
 
-	if _, ok := items[0].(stackitem.Null); ok {
-		return nil, ErrConfigNotFound
-	}
-
-	return assert(items[0])
-}
-
-// IntegerAssert converts stack item to int64.
-func IntegerAssert(item stackitem.Item) (any, error) {
-	return client.IntFromStackItem(item)
-}
-
-// StringAssert converts stack item to string.
-func StringAssert(item stackitem.Item) (any, error) {
-	return client.StringFromStackItem(item)
-}
-
-// BoolAssert converts stack item to bool.
-func BoolAssert(item stackitem.Item) (any, error) {
-	return client.BoolFromStackItem(item)
+	return items[0], nil
 }
 
 // iterateRecords iterates over all config records and passes them to f.